This invention relates to filter elements and to improved methods and structures for providing end and side seals therein.
Filter elements in a variety of materials and structures are commercially available and examples are described in U.S. Pat. Nos. 2,732,031; 3,013,667; 3,457,339; and 3, 850,813. As illustrated therein, a sheet of filter material is formed into a cyclinder or the like, often with pleats to increase the filter area, and the side edges and ends sealed. The ends are commonly sealed by potting in solidified liquid sealers such as epoxy or polyurethane resins, molten thermoplastics, or the like. The elements are then mounted in holders which provide connections to supply fluid to be filtered to the upstream side of the filter material and to remove filtrate from the downstream side. Commonly, cylindrical elements include a hollow internal porous support of metal or plastic, fluid is supplied to the exterior of the filter material, and filtrate is removed from the internal support.
For filtering fine particulates, thin fibrous or cast microporous membranes having a pore size from about 0.1 to 25 microns are used which have about 50-80% of their volume as voids. Such membranes are relatively weak and fragile, especially when pleated. Coarser permeable layers on one or both sides thereof are often used for support and to maintain flow channels between pleated membrane plies. Even when supported, however, failures occasionally occur at one of the seals, particularly at the upstream junction of the end sealer and membrane.
For use in the sterilization of liquids, or in other critical applications, completed filter elements must be tested for integrity prior to use. This is done, for example, by filling the membrane pores with a liquid, typically water, and measuring the flow rate of a gas, typically air, through the element under a pressure which is a substantial portion of the membrane bubble point, for example, 80%. For a given membrane, the bubble point and diffusion rates are known, and any leaks will produce a substantially increased flow.
A number of procedures have been employed to improve the reliability of the end seals for tubular filter elements. Shallow, densely packed pleats for mutual reinforcement have been used. An extra layer of porous membrane has been employed at the ends. A strip of flexible plastic, for example a flexible polyurethane, has been applied to the membrane for reinforcement at the junction of membrane and sealer. None has been entirely satisfactory, especially for use with fragile cast microporous membranes in critical applications. The use of flexible reinforcing strips introduces additional materials and steps, reduces the effective filtering area, and increases the sources for extractables into the filtrate.